Hydrothermal performance evaluation of parabolic trough collector with conical dimpled receiver

Abstract

Abstract There is an extraordinary urgency to limit hydrocarbon pollution and CO2 emissions. Due to their high temperatures and flow rates, parabolic trough collectors (PTCs) are used in industry to harness solar energy. Various studies have been carried out to improve PTC efficiency and performance. Multiple methods have been suggested; however, improving the transfer of heat between the absorber tube and heat transfer fluid (HTF) has the most impact. The current investigation introduces a new geometric modification for the absorber tube by utilizing four different inner dimples with and without conical tubes to promote thermal–hydraulic performance. The parametric study is conducted numerically using Ansys Fluent in combination with TracePro under a wide range of Reynolds number $$(Re)$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>(</mml:mo> <mml:mi>R</mml:mi> <mml:mi>e</mml:mi> <mml:mo>)</mml:mo> </mml:mrow> </mml:math> between 20,000 and 100,000 to assess the thermal performance, i.e., Nusselt number $$(Nu)$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>(</mml:mo> <mml:mi>N</mml:mi> <mml:mi>u</mml:mi> <mml:mo>)</mml:mo> </mml:mrow> </mml:math> , thermal efficiency $$({\eta }_{\text{th}})$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>(</mml:mo> <mml:msub> <mml:mi>η</mml:mi> <mml:mtext>th</mml:mtext> </mml:msub> <mml:mo>)</mml:mo> </mml:mrow> </mml:math> , and exergetic efficiency $${(\eta }_{\text{exe}})$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mo>(</mml:mo> <mml:mi>η</mml:mi> </mml:mrow> <mml:mtext>exe</mml:mtext> </mml:msub> <mml:mrow> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:math> . In addition, the friction coefficient $$(f)$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>(</mml:mo> <mml:mi>f</mml:mi> <mml:mo>)</mml:mo> </mml:mrow> </mml:math> has also been evaluated to demonstrate the hydraulic performance. Lastly, the combination of both the thermal and hydraulic performance is also reported through the thermal enhancement factor $$(TEF)$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>(</mml:mo> <mml:mi>T</mml:mi> <mml:mi>E</mml:mi> <mml:mi>F</mml:mi> <mml:mo>)</mml:mo> </mml:mrow> </mml:math> , adjusted thermal $${(\eta }_{\text{th}-\text{adj}.})$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mo>(</mml:mo> <mml:mi>η</mml:mi> </mml:mrow> <mml:mrow> <mml:mtext>th</mml:mtext> <mml:mo>-</mml:mo> <mml:mtext>adj</mml:mtext> <mml:mo>.</mml:mo> </mml:mrow> </mml:msub> <mml:mrow> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:math> , and exergetic $${(\eta }_{\text{exe}-\text{adj}.})$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mo>(</mml:mo> <mml:mi>η</mml:mi> </mml:mrow> <mml:mrow> <mml:mtext>exe</mml:mtext> <mml:mo>-</mml:mo> <mml:mtext>adj</mml:mtext> <mml:mo>.</mml:mo> </mml:mrow> </mml:msub> <mml:mrow> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:math> efficiencies. It was found that despite the promising enhancement of heat transfer associated with the dimples in conical tubes starting from 80% up to 150%, the friction coefficient leaped significantly, ranging between 400 and 900%, and even recorded six times that percentage in certain cases depending mainly on the dimple design and $$Re$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>Re</mml:mi> </mml:mrow> </mml:math> . The $$TEF$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>TEF</mml:mi> </mml:mrow> </mml:math> showed that the dimpled tubes with a slight convergence of the outlet diameter (DR = 1.25) operated effectively at low $$Re$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>Re</mml:mi> </mml:mrow> </mml:math> . Besides, it also highlights that the best dimple design to use the stepped-conical dimple as the overall thermal–hydraulic performance increased over 20% at $$Re$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>Re</mml:mi> </mml:mrow> </mml:math> equal to 20,000. On the other hand, the smooth absorber tube with a DR = 1.25 showed the exact outcome at $$Re$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>Re</mml:mi> </mml:mrow> </mml:math> exceeding 80,000. Finally, a comparison with the previously proposed techniques has also been conducted. Clearly, the dimpled and smooth conical perform well in their categories. In conclusion, the study offers two promising designs to increase PTC thermal performance at low and high mass fluxes for a variety of industrial applications.

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